1. Field of the Invention
The present invention relates to a vacuum degassing system and more particularly pertains to a method and apparatus associated with removing gases from liquids in a flow-through relation in which an elongated gas-permeable tube addresses a vacuum chamber evacuated by a variable speed vacuum pump and gas is transferred by diffusion through the walls of the tube. The system is particularly suited to the removal of air or oxygen from the mobile phase associated with high performance liquid chromatographic equipment.
2. Description of the Prior Art
There are many chemical applications, particularly analytical applications, involving the use of liquid solvents, reactants or the like in which the presence of dissolved gases, particularly air, is undesirable. A prime example of such an application relates to the mobile phase in high performance liquid chromatography where the presence of even small amounts of dissolved gases, and in particular oxygen, interferes with the accuracy and sensitivity of the results obtained. For example, air dissolved in the mobile phase can manifest itself in the form of bubbles which causes noise and drift as the mobile phase passes through the detector. If the dissolved species be chemically active, as in the case of oxygen in air, it can additionally produce unwanted changes or deterioration in the mobile phase. Of course, the detrimental effect of the dissolved species is related to the relative concentration of the species in the mobile phase. These undesirable species usually are removed by a degassing process. It correspondingly follows that the more efficient the removal or degassing system is, the more desirable it will be.
The degassing of liquid materials has been necessary to the success of many processes and, consequently, the process has been pursued actively in several forms for a long period of time. Techniques have included heating or boiling the liquid to be degassed, exposing the material to a reduced pressure environment or vacuum and using combinations of heat and vacuum to reduce the amount of dissolved gases in the liquid. Ultrasonic energy has also been employed. As conventionally applied, however, these traditional techniques have generally fallen short of the desired degree of separation efficiency. Additionally, a means of degassing solvent involving the passing of a fine stream of bubbles of inert gas such as helium through the solution to be degassed has been shown by Bakalyar et al. in U.S. Pat. No. 4,133,767, and in apparatus such as that disclosed by Sims et al. in U.S. Pat. No. 4,994,180, co-invented by the co-inventor in the present application and assigned to the same assignee as the present invention.
Vacuum degassing through a membrane apparatus has long been known, and generally utilizes a length of relatively small diameter, thin-walled semi-permeable synthetic polymer resin material contained within an enclosed chamber held under a reduced pressure or vacuum in which the liquid to be degassified is caused to flow through the tube. One such apparatus is shown by Sims in U.S. Pat. No. 5,340,384, co-invented by the co-inventor in the present application and assigned to the same assignee as the present invention. Other such devices are shown in U.S. Pat. Nos. 5,183,486, 4,430,098, and 3,668,837.
While each of these devices employ a flow-through tube vacuum degassing approach, there remains a need, particularly with devices associated with high performance liquid chromatography instruments, to make degassing of solvents, and in particular the mobile phase, more efficient. One particular limitation or drawback associated with present devices concerns the efficiency of the degassification operation with respect to the composition of the tubing itself. Materials presently used in degassing applications include PTFE, PFA, and silicone rubber. These materials, while generally suitable for this application, require that the wall thickness be as thin as possible due to the gas permeability of materials typically utilized for these applications. A large internal diameter tube is disadvantageous as the gas must diffuse through a longer path from the center of the flow to the inner wall surface, thereby requiring a long tube. Additionally, a tube of greater length increases flow resistance through the overall system the resistance being a linear function of tubing length (assuming laminar liquid flow through the tubing). Liquid flow resistance is an inverse function of tubing internal diameter to the fourth power.
Amorphous perfluorinated copolymers reportedly have permeabilities of up to 2 or 3 orders of magnitude higher than those of PTFE. It has been found by the present inventors that by using amorphous perfluorinated copolymers, such as those marketed by Du Pont under the tradename Teflon AF that permeabilities of up to about 1 order of magnitude or greater are experienced. Nevertheless, in the fabrication of degassing tubes, greater gas mass transfer rates can be achieved with tubes of Teflon AF having increased wall thicknesses, thereby permitting the undertaking of applications requiring higher pressures. Advantageously, tubes of smaller internal diameter and shorter length offer reduced internal volumes. Low flow resistance is accomplished with multi-lumen tubing arrangements.
Because of the enhanced gas permeability property of materials utilized in accordance with the present invention, the diffusion rate of atmospheric gases from the liquid being degassed through the tubing wall is significantly increased. It appears likely that the increased gas permeability is enhanced by the free (void) volume in the polymer component.
As a further feature of this invention, it has been found that very stable reduced pressure or vacuum is achieved within the vacuum chamber. This feature is possible due to the operational characteristics of the vacuum pump. In initial operation, the pump (typically operated @ 400 RPM) reduces pressure inside the vacuum chamber. When the pressure inside the chamber begins to asymptotically approach a maximum differential value (typically around 60 mm Hg absolute) the speed is substantially reduced, such as to bout 60 RPM. The pump is run continuously at this reduced rate, with the vacuum then slowly descending to a xe2x80x9cconstant vacuum levelxe2x80x9d with the pressure remaining constant for so long as the pump is running. This xe2x80x9cconstant vacuum levelxe2x80x9d provides significant advantages in that it eliminates vacuum (pressure) hysteresis which typically is in the range of 15-25 mm Hg as the result of cycling the pump on and off as in other systems. Through this operational feature, variations in remaining atmospheric gas in the mobile phase exiting the degassing apparatus to the liquid chromatograph are also eliminated. This feature provides technical advantages because of the resulting HPLC detector base line stability. Superior vacuum level, typically in the range of 30 mm Hg or less, also reduces the absolute concentration of dissolved gases in the mobile phase, which improves the flow rate precision of the HPLC pump. In addition, longer life expectancy of the vacuum pump is achieved because of low RPM.
Accordingly, it is a principal object of the present invention to provide a more efficient vacuum degassing system of the flow-through type using a tube or multiple tubes formed from an amorphous perfluorinated copolymer.
A further object of the present invention is to reduce the required inside diameter and length of the degassing tube.
A still further object of the present invention is the provision of a tube having a single lumen.
A yet further object of the present invention is the provision of a tube having multiple lumen.
Another object of the present invention is the provision of a variable speed pump which is run continuously for evacuating the vacuum chamber, with the effects being a reduction or elimination of hysteresis and increased vacuum pump life expectancy.
Still further object of the present invention is the utilization of a flow restrictor in the system to facilitate bleeding or flushing of the head with atmosphere to prevent solvent vapor build up.
Still another object of the present invention is to provide a means for interconnecting liquid chromatography instrument components which simultaneously degasses the mobile phase in transit between the components.
It is yet a further object of the present invention to provide an improved vacuum pump head bleed or flush system in the form of a vent frit including a sintered metal plug, thereby eliminating the necessity of the solenoid operated vent valve typically utilized in current systems.
A still further object of the invention is to provide an improved connection system for the tube associated with a flow-through vacuum degassing apparatus.
By means of the present invention, the efficiency of a flow-through vacuum degassing system utilizing an elongated gas-permeable tube is improved by reducing the required inside diameter and length of the tube. This is achieved by forming the tube from an amorphous perfluorinated copolymer such as Teflon AF. Amorphous perfluorinated copolymers have been reported to have permeabilities of up to 2 or 3 orders of magnitude greater than other semipermeable polymeric resins utilized in degassing applications. By using such copolymers, it has been found that it is possible to significantly reduce the length of tubing utilized, which correspondingly and proportionally reduces the internal volume. All of this is achieved without either reduction or compromise in degassing performance.
Gas mass transfer rates are further improved by the present invention through the use of a tube having multiple lumen. The multiple lumen tubing arrangement offers greater degassing efficiency by providing a greater surface area through which the gas may travel and a smaller tubing diameter. A multiple lumen tubing of smaller diameter provides for reduced internal volume and lower flow resistance to the mobile phase sought to be degassed.
The degassing chamber of the invention includes an injection-molded plastic housing which is preferably sealed with o-rings or other sealing devices. The chamber is provided with a vacuum connection and liquid inlet and outlet connections for a coil of gas-permeable tubing. The coil may be either a single lumen tube or a multiple lumen tube. A pair of interface grids each positioned between a bulkhead fitting and an inlet or outlet nut is fabricated of TEFZEL(copyright), KEL-F, PTFE or PEEK for use with the multiple lumen tube and includes a center bore and a plurality of radially spaced bores for sealingly receiving the tubes in an adhesiveless manner when they are pulled through during assembly of the degassing chamber. Teflon AF tubing may be advantageously utilized, when placed through a slightly over-sized hole, with the tubing being pushed through and compressed on the ferrule with a nut. Compression seals utilizing TEFZEL(copyright) ferrules have been found highly useful and are preferred in creating seals without requiring adhesives.
In an alternative embodiment of the invention, a degassing transfer line for interconnecting liquid chromatography system components includes a length of Teflon AF tubing extending between opposed ends of the transfer line and disposed within an elongate tube formed of an adhesive-lined, heat shrinkable material, with this alternative embodiment being described in detail herein. In this alternative embodiment, opposed ends of the tube sealingly surround a PTFE/FEP dual-shrink tubing through which the Teflon AF tubing extends. Distally of each opposed end of the elongate tube, a nut is sealingly attached to the PTFE/FEP tubing. Distally of the nut, ferrules are provided for connection to the various LC components. A vacuum adapter is provided for communication between an interior of the elongate tube and a vacuum source to evacuate the interior of the elongate tube and thereby degassing the mobile phase as it flows through the Teflon AF tubing.
Another feature of the present invention provides for a variable speed vacuum pump which evacuates the vacuum chamber. In a first preferred mode of operation, electronic control means responsive to a sensed vacuum level are operable to drive a variable-speed motor which in turn drives an eccentric shaft coupled to a two-stage diaphragm pumping mechanism at a high speed to quickly evacuate the vacuum chamber (400 RPM) and at a low speed, such as about 60 RPM, for continuous operation and long life of the degassing system. In an alternative second mode of operation, a vacuum setpoint is set and the pump is intermittently driven at the high speed when the sensed pressure rises above the setpoint, with the rate dropping once the desired pressure reduction has been achieved. In an alternative third mode of operation, a sensed vacuum level is continuously compared to a vacuum setpoint to provide electronic control means with specified input to continuously control pump rpm so as to maintain a constant vacuum.